Currently, low temperature polysilicon is mainstream for TFTs in mid-to small-sized liquid crystal display devices, but low temperature polysilicon has a small grain size, low mobility, and varies a lot in TFT characteristics. In order to improve TFT characteristics, single-crystal silicon (Si) with no crystal grain boundary may be formed, but it is difficult to form single-crystal silicon on top of a large substrate using a regular film forming method. A technology to transfer a single-crystal silicon wafer to a top of a glass substrate is being studied (see Non-Patent Documents 1 and 2).
Furthermore, while efforts have been made in improving TFT performance, the development of a flexible display that has a curved surface and flexible characteristics has also been taking place (see Non-Patent Document 3). In order to realize a flexible display, a plastic substrate such as PET can be used as a substrate that is flexible, but a regular plastic substrate only has an upper temperature limit of approximately 100° C., so the TFT characteristics decrease. Furthermore, if a polyimide (PI) or the like is used as a flexible substrate, then the upper temperature limit is approximately 400° C., so low temperature polysilicon can be formed, but it is still not enough to obtain high TFT performance.
Furthermore, if a single-crystal silicon film can be transferred to the top of the flexible substrate, then even higher performing flexible displays can be realized, but for transferring the single-crystal silicon wafer, heat treatment of over several hundred degrees Celsius is necessary. However, the coefficient of thermal expansion of a plastic substrate made of PET, PI, or the like and a silicon wafer is highly different, and the plastic substrate bends during heat treatment, and thus the single-crystal silicon wafer cannot be reliably transferred.
Recently, a thin film glass with a thickness of 100 um or less that can be bent has been reported (Non-Patent Document 4), but such a thin glass substrate is easy to break and is very difficult to wash or transport, and thus is difficult to use in the actual manufacturing process.
Patent Document 1 discloses a method (see FIG. 13) to bond a silicon wafer and a flexible substrate formed of glass, plastic, or the like having flexible characteristics, but even with this method, problems such as transferring being insufficient due to the substrate tearing off because of the difference in the coefficient of thermal expansion, and damaging of the glass substrate, and the like may occur.